To study the role of vasoactive substances which act as mediators of inflammation (histamine, bradykinin, prostaglandin E, serotonin etc.) in the afferent and efferent arcs of the immune response to tissue and organ transplantation with the heterotropic (abdominal aorta) heart transplant in the rat as the major model. Metabolic alterations regarding those substance and the expression of receptors on sensitized lymphocytes (by a variety of assays) are major parts of this study. In allograft rejection an inflammatory response appears to play an important part as in progressive tumor growth there appears to be an anti-inflammatory component. The role of inflammation and mediators of inflammation in these two important expressions of the cellular immune response (transplant and tumor) are worthy of further study.